System for and method of capturing application characteristics data from a computer system and modeling target system

ABSTRACT

A system for, method of and computer program product captures performance-characteristic data from the execution of a program and models system performance based on that data. Performance-characterization data based on easily captured reuse distance metrics is targeted, defined as the total number of memory references between two accesses to the same piece of data. Methods for efficiently capturing this kind of metrics are described. These data can be refined into easily interpreted performance metrics, such as performance data related to caches with LRU replacement and random replacement strategies in combination with fully associative as well as limited associativity cache organizations.

RELATED APPLICATIONS

The present application claims the benefit of the earlier filing date of U.S. Provisional Patent Application No. 60/810,622 filed Jun. 5, 2006 in accordance with 35 U.S.C. §119(e).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present application relates in general to systems, methods and software for analyzing, parallelizing, debugging, optimizing and profiling computer systems and, more specifically to capturing application characteristic data from the execution of a system or multiple systems, and modeling system behavior based on such data.

2. Description of the Related Art

Increasing demands to improve software efficiency with ever-increasing system complexity have dictated the use of tools to evaluate target software operation, identify inefficiencies, suggest and/or implement improvements, optimize software operation, etc. Optimization and profiling tools often embed monitoring code into the target software under scrutiny and/or create a real or simulated (e.g., model) run-time environment that interacts with the target software to analyze its operation. See, for example, “StatCache: A Probabilistic Approach to Efficient and Accurate Data Locality Analysis”, E. Berg and E. Hagersten, Technical report 2003-58 Dept. of Information Technology, Uppsala University, Uppsala, Sweden, November 2003, Proceedings of the 2004 IEEE International Symposium on Analysis of Systems and Software (ISPASS-2004), Austin, Tex., USA, March 2004; “Low Overhead Spatial and Temporal Data Locality Analysis”, E. Berg and E. Hagersten, Technical report 2003-57 Dept. of Information Technology, Uppsala University, Uppsala, Sweden, November 2003; and, A Statistical Multiprocessor Cache Model by Erik Berg, Håkan Zeffer, and Erik Hagersten. In Proceedings of the 2006 IEEE International Symposium on Analysis of Systems and Software (ISPASS-2006), Austin, Tex., USA, March 2006, each of which is incorporated herein by reference in its entirety.

Other publications applicable to the related technology include:

-   -   STATSHARE: A Statistical Model for Managing Cache Sharing via         Decay by Pavlos Petoumenos, Georgios Keramidas, Håkan Zeffer,         Stefanos Kaxiras, and Erik Hagersten. In 2006th Workshop on         Modeling, Benchmarking and Simulation held in conjunction: with         the 33rd Annual International Symposium on Computer         Architecture, Boston, Mass. USA, June 2006, 2006.     -   Modeling Cache Sharing on Chip Multiprocessor Architectures by         Pavlos Petoumenos, Georgios Keramidas, Håkan Zeffer, Erik         Hagersten, and Stefanos Kaxiras. In Proceedings of the 2006 IEEE         International Symposium of Workload Characterization: San Jose,         Calif., USA, 2006.

An ideal profiling tool should have low run-time overhead and high accuracy, it should be easy and flexible to use, and it should provide the user with intuitive and easily interpreted information. Low run-time overhead and high accuracy are both needed to efficiently locate bottlenecks with short turn-around time, and the ease-of use requirement excludes methods which need cumbersome experimental setups or special compilation procedures.

It is unfortunately hard to combine all the requirements above in a single method. For example methods based on hardware counters usually have a very low run-time overhead, but their flexibility is limited because hardware parameters like cache and TLB sizes are defined by the host computer system. Simulators on the other hand are very flexible but are usually slow. At worst, they may force the use of reduced data sets or otherwise unrepresentative experiment setups that give misleading results.

There are a variety of methods to perform cache behavior studies. These include simulation, hardware monitoring, statistical methods and compile-time analysis. Compile-time analysis tools [35][8] estimate cache miss ratios by statically analyzing the code and determine when cache misses occur. Compile-time analysis major advantage is that it doesn't require the program to be executed, and can potentially be parameterized in terms of workloads etc. Its drawback is that it is limited to relatively well-structured codes where for example loop limits are known at compile time.

Cache simulators may be driven by instrumented code [13, 14, 20, 21, 23, 26, 27], on source code [17] or machine code levels, or the cache simulator incorporated in a full system simulator [24][22]. Their major limitation is their large slowdown. Simulation-based analysis can possibly combined with sampling (see below) to reduce the runtime overhead.

Cache-sampling techniques include set sampling and time sampling. In time sampling a cache model simulates continuous sub-traces from the complete access trace.

This is explored in papers [11, 15, 18, 19, 36]. It works well for smaller caches, but the need for long warm-up periods makes time sampling less suitable for large caches. The problem of selecting statistically representative samples is explored in Perelman et al.[32] Set sampling is another approach, were only a fraction of the sets in a set-associative cache is simulated [11, 18]. It generally suffers from poor accuracy and can only be used as a rough estimate.

More recently, sampling guided by phase detection has been proposed [31, 37]. The idea is based on the observation that most applications have different phases during their execution. Within each phase, the system performs in a fairly invariant (often repetitive) way. Guided by phase detection algorithms, very sparse samples can still provide a representative behavior for the entire execution. While most work on phase-guided sampling has been targeting detailed pipeline simulation, similar techniques could also be applied to memory system modeling. Cutting down the number of samples for time-sampling of caches could turn out to be especially valuable, since the need to warm the large caches requires so many memory operations per sample. Phase detection could also work well together with our tool. Phase detection could guide us to sample more or less often during the execution which could cut back on out runtime overhead further. The fact that we do not need to warm the caches before our model is valid further speaks in our favor.

Hardware counters are available on most modern computers. Events that can be counted include L1 and L2 cache misses, coherence misses and number of stall cycles. Examples of use include DCPI [1], which uses an advanced hardware support to collect detailed information to the programmer, PAPI [6] which is a common programming interface to access hardware monitoring aids, and several commercial tools [12, 16]. Histogramming and tracing hardware may be used to detect for example cache conflicts [30] and locate problem areas [7]. Their limitations are mainly that only architectural parameters realized on the hardware may be studied, and that it can be hard to capture entities not directly present in the hardware, such as spatial locality. Trap-driven trace generation has also been suggested [34]. It can trace unmodified code, but requires OS modification.

Other approaches to describe and quantify memory behavior include the concept of data streams or strides. Information about data streams can be used to guide prefetching [9][10] and help choose between optimizations such as tiling, prefetching and padding[29]. Abstract cross-platform models for analyzing and visualizing cache behavior exist [25, 5, 38], mostly based on a reuse distance definition similar to the stack distance [28].

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BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention are directed toward computer system measuring techniques used for estimating the behavior of some application running on a specific piece of hardware or hardware platform. Another object of embodiments of the invention includes identifying a specific problem related to a specific piece of software with respect to one or many different hardware architectures, or to find opportunities to dynamically adjust the functionality of the system to avoid some problems.

More specifically, systems, methods and techniques according to various aspects of the invention may be directed toward targeting efficient techniques for capturing representative characteristics data associated with a piece of software and to using this data for modeling computer hardware in a way that can translate that characteristic into measurements (e.g., “numbers”) and/or for identifying problems and opportunities.

Thus, an object of the invention is to provide a tool for the evaluation software that can be used to enhance of the target software. One embodiments of the invention focus on such parameters as Memory Access and identify ways to predict cache misses and their associated slow Memory Access. A preferred embodiment of the invention is a software profiling tool. All forms of caches, including but not limited to disk caches, web caches, the caching of virtual memory to physical memory, TLB translation caches, and the caching functionality of branch target buffers, can be targeted using the described techniques, even though the examples used herein are for ease of illustration alone, centered around caches used in memory systems.

According to one aspect of the invention, embodiments employ statistical modeling as an alternative to or in addition to hardware-counter-based and simulation-based approaches. Such methods have the potential to satisfy demands for flexibility, speed and ease-of-use.

Further, methods according to various embodiments of the invention can be parameterized since they do not rely on the host hardware configuration, are potentially very fast because they can be based on sparse sampling and can also provide a simple and efficient user environment.

Further aspects, features, objectives and advantages of the present invention can be found in the publications “Fast Memory-Locality Profiling of Native Execution”, E. Berg and E. Hagersten, SIGMETRICS '05, June 6-10, Banff, Alberta, Canada and “A statistical Multiprocessor Cache Model”, E. Berg, H. Zeffer and E. Hagersten, Uppsala University, Department of Information Technology, Uppsala, Sweden, March, 2006, attached to the present disclosure as Appendices A and B, respectively.

According to one aspect of the invention emphasizing a sampling feature, a method of gathering memory usage information may include the steps of starting the execution of a sequence of operations in response to a program of instructions; selecting a set of the operations to be monitored; recording access information associated with respective operations of the set of operations and identifying one or more memory objects; enabling a watchpoint mechanism to detect one or more next operations accessing one or more of the memory objects; and recording information about the next operations. The program of instructions may be any set of executables, commands, directives, etc. that may be read and control the operation of a computer or other programmable apparatus.

The step of selecting may be accomplished randomly, implement some set of rules, or otherwise target certain types, classes, categories and/or specified operations to be monitored. For example, operations targeted and selected for monitoring may be those (i) requiring some threshold number of machine cycles to execute and/or memory accesses; (ii) subject to some threshold value of repeated executions such as present within an inner loop of a program; (iii) identified as a bottleneck to the execution of multiple subsequent instructions; and/or having some other characteristic that may be relevant to program performance.

According to a feature of the invention, the step of storing access information may further include storing an identity of the one of more memory objects being accessed. The identity may be a relative or absolute memory address of the memory object, a variable name associated with the memory object, a link referencing the memory object, a value or variable name stored in or associated with the memory object, etc.

According to another feature of the invention, the step of enabling a watchpoint mechanism may include enabling a hardware device to provide a signal indicative of the next operations accessing the one or more of the memory objects. This may include using a breakpoint register or some other device that is not directly used by the program and storing a breakpoint corresponding to the one or more of the memory objects in the breakpoint or other register. The hardware device may include logic for detecting an attempted access to a designated portion of or location in memory and, as a result, generating an interrupt such as a hardware interrupt signal, etc.

According to another feature of the invention, the step of enabling a watchpoint mechanism may include enabling an externally provided trap mechanism to detect the one or more next operations accessing the one or more of the memory objects. The externally provided trap mechanism may be in the form of software, such as a shell or operating system, and/or hardware, e.g., a breakpoint register and related logic.

According to another aspect of the invention including a cold miss feature, a method of estimating the amount of memory objects used by a programs sequence may include the steps to gather memory usage information followed by followed by the steps of estimating the fraction of the monitored instruction for which no corresponding next instruction was detected; and multiplying that fraction with the total number of operations executed by the program sequence.

The step of estimating the fraction of monitored instructions for which no corresponding next instruction may be performed by assessing the number of initiated watchpoint mechanisms that still have not found any other instruction accessing the same memory object as the corresponding selected operation when the end of the program sequence is reached.

According to another feature the memory space used by the program may be estimated by identifying the monitored instruction for which no corresponding next instruction was detected and compute the sum their memory objects sizes.

According to another aspect of the invention including a random replacement feature, a method of estimating cache miss rate for a cache system including the steps of: executing a sequence of accesses to a memory system; recording reuse distance information for a selected fraction of memory accesses of this sequence; and solving an unknown miss rate variable from an equation equating the total number of memory accesses multiplied by the unknown miss rate, with the number of unique memory objects in the sequence plus the sum of the miss probabilities for the selected of memory accesses divided by the fraction of memory accesses for which reuse distance information was recorded.

Reuse distance for one memory operation may be measured as the number of memory operations that have been performed since the memory object it accesses was last accessed. The number of unique memory objects accessed in the sequence may be estimated by the number of cold misses in the sequence.

According to another aspect of the invention incorporating stack distance features, a method of estimating a number of unique memory objects touched for a segment of a program of instructions include the steps of; executing the program and estimating a representative reuse distance distribution for memory operations of the studied segment; estimating the amount of memory operation performed during the segment; for each number between that amount and one, use the distribution to calculate the probability a memory operation of has a reuse distance larger than the number; and add all those probabilities together. The number of unique memory object touched is sometimes referred to as stack distance.

A representative reuse distribution may be comprised of the a histogram based on reuse distance from a randomly chosen set of memory operations executed during, or reasonably close to, the studied segment. In some situations, the reuse distribution of memory operations collected during the entire execution may be sufficient for this task. For each bucket in the histogram, the population number recorded is divided by the fractions between the number of randomly chosen memory operations and the total number of memory operations executed during the timeframe from which the random selection was made.

In one feature of the invention the ability to estimate the number of unique memory objects accessed during a segment of the execution may be used in the following steps to estimate the miss rate of an LRU cache: finding the smallest reuse distance value for which the number of unique memory object is larger than the number of memory objects that can be stored in the cache; and calculating the fraction of memory operations of said reuse distribution with a reuse value larger than said number.

According to another aspect of the invention, an apparatus may provide the functionalities and perform the methods and steps according to the above described features and aspects of the invention as further set forth below by way of example.

Thus, according to another aspect of the invention, an apparatus for gathering memory usage information may include a processor operating to execute a sequence of operations in response to the program; selecting logic for selecting a set of operations from the sequence of operations; monitoring logic for monitoring respective operations of the set of operations; a first memory storing access information associated with respective operations of the set of operations including an identify one or more memory objects being accessed by respective ones of the set of operations; a watchpoint mechanism operable to detect one or more next operations accessing one or more of the memory objects; and a second memory storing access information about the next operations. The processor may be part of, for example, a conventional computer such as a personal computer, workstation, mainframe, etc, capable of reading and executing the program of instructions. The selecting and/or monitoring logic may be software, firmware, and/or hardware, or any combination thereof capable of performing or causing to be performed the recited steps and/or operations. The first and second memories may be the same or different portions of one or more physical or virtual devices, e.g., registers or memory locations, etc.

According to a feature of the invention, the selecting logic may be further operable to enable an imprecise hardware trap with a maximum skid of S operations to signal S operations before one of the next operations is executed, and to advance the execution using a technique having less skid, until the one next operation is reached.

According to another feature of the invention wherein the first memory stores the access information about the selected set of operations, such information may include one or more pieces of information selected from the group consisting of: (i) identities of the selected operations; (ii) identities of the memory objects; (iii) memory addresses being accessed; (iv) identities of memory operations following next after respective ones of the selected set of operations; and (v) identities of memory operations preceding before respective ones of the selected set of operations.

According to another feature of the invention wherein the first memory stores the access information about the next operations, such information may include one or more pieces of information selected from the group consisting of: (i) identities of the next operations; (ii) identities of the memory objects; (iii) memory addresses being accessed; (iv) identities of memory operations following next after respective ones of the next operations; and (v) identities of memory operations preceding before respective ones of the next operations.

According to another feature of the invention, the apparatus may further include estimating logic (e.g., programming, software, hardware, firmware, or other means for performing or causing to be performed the recited steps or processes) for estimating an amount of memory objects used by the sequence of instructions including estimating logic for estimating a fraction of the monitored instructions for which no next instruction is detected; and a multiplier for multiplying the fraction with a total number of the operations executed by the sequence of instructions.

According to another feature of the invention, the apparatus may further include estimating logic for estimating an amount of memory space used by the sequence of operations including identifying logic for identifying the monitored instruction for which no the next instruction is detected; and an adder for summing each memory object size accessed for each the identified instruction to form a total.

According to another feature of the invention, the apparatus may further include estimating logic for estimating an amount of cache space used by the sequence of operations wherein a memory object size is equal to a cache line size of the cache, the step of estimating further including estimating logic for estimating a fraction of the monitored instruction for which no next instruction is detected; and a multiplier for multiplying the fraction with a total number of the operations executed by the program sequence and the memory object size of the cache.

According to another feature of the invention, an apparatus for estimating a cache miss rate for a cache system may include a processor for executing a sequence of accesses to a memory system; a memory storing reuse distance information for a selected fraction of the memory accesses of the sequence; and logic for solving an unknown miss rate variable from an equation equating the total number of memory accesses multiplied by the miss rate, with a number of unique memory objects in the sequence plus a sum of the miss probabilities for the selection fraction of memory accesses divided by the fraction.

According to another feature of the invention, an apparatus for estimating a number of unique memory objects accessed for a segment of a program of instructions may include a processor for executing the program and estimating a representative reuse distance distribution for memory operations of the segment; estimating logic for estimating an amount of memory operations performed during the segment; logic using the reuse distance distribution to calculate, for each natural number between the amount and the number one, a probability a memory operation of the distribution has a reuse distance larger than the natural number; and an adder for adding all the probabilities together.

According to another aspect of the invention, a computer readable medium may include code including instructions for implementing and/or performing the methods, steps and actions set forth above. Thus, by way of example, according to another aspect of the invention, a computer readable medium may include code for gathering memory usage information, the code including instructions for executing a sequence of operations in response to a program of instructions; selecting a set of the operations to be monitored; storing access information associated with respective operations of the set of operations including an identity of corresponding operations accessing each of one or more memory objects; enabling a watchpoint mechanism to detect one or more next operations accessing one or more of the memory objects; and recording information about the next operations. The computer readable medium may be, for example, random access memory, read only memory, permanent and removable magnetic and/or optical storage devices and media, etc.

According to another aspect of the invention, a computer readable medium may include code for gathering memory usage information, the code including instructions for starting to execute a sequence of operations in response to the program; selecting a set of operations from the sequence of operations; monitoring respective operations of the set of operations; recording access information associated with respective operations of the set of operations and identifying one or more memory objects being accessed by respective ones of the set of operations; enabling a watchpoint mechanism operable to detect one or more next operations accessing one or more of the memory objects; and recording access information about the next operations.

According to a feature of the invention, the code for selecting a set of operations further may include code for enabling an imprecise hardware trap with a maximum skid of S operations to signal S operations before one of the next operations is executed, and to advance the execution using a technique having less skid, until the one next operation is reached.

According to another feature of the invention, code may be included for estimating an amount of memory objects used by the sequence including instructions for estimating a fraction of the monitored instructions for which no next instruction is detected; and multiplying the fraction with a total number of the operations executed by the sequence of instructions.

According to another feature of the invention, code may be included for estimating an amount of memory space used by the sequence including instructions for identifying the monitored instruction for which no the next instruction is detected; and summing to form a total of each memory object size accessed for each the identified instruction.

According to another feature of the invention, code may be included for estimating an amount of cache space used by the sequence of operations wherein a memory object size is equal to a cache line size of the including instructions for estimating a fraction of the monitored instruction for which no next instruction is detected; and multiplying the fraction with a total number of the operations executed by the program sequence and the memory object size of the cache.

According to another aspect of the invention, a computer readable medium may include code for estimating a cache miss rate for a cache system, the code including instructions for executing a sequence of accesses to a memory system; recording reuse distance information for a selected fraction of the memory accesses of the sequence; and solving an unknown miss rate variable from an equation equating the total number of memory accesses multiplied by the miss rate, with a number of unique memory objects in the sequence plus a sum of the miss probabilities for the selection fraction of memory accesses divided by the fraction.

According to another aspect of the invention, a computer readable medium may include code for estimating a number of unique memory objects accessed for a segment of a program of instructions, the code including instructions for executing the program and estimating a representative reuse distance distribution for memory operations of the segment; estimating an amount of memory operations performed during the segment; for each natural number between the amount and the number one, using the reuse distance distribution to calculate a probability a memory operation of the distribution has a reuse distance larger than the natural number; and adding all the probabilities together.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a block diagram of a system for gathering characteristics of a program using a sampler method and wherein the captured characteristics may be used as input to a system model;

FIG. 2 is a diagram illustrating a sparse selection of samples chosen from a stream of Memory Accesses and examples of different types of characteristic data that may be recorded;

FIG. 3 is a block diagram of a prior art memory system including main and cache memories;

FIG. 4 is a diagram of a fully associative cache illustrating how a random replacement may be model;

FIG. 5 is a diagram of a stream of Memory Accesses illustrating a method of estimating a miss probability for each access in a stream of Memory Accesses based on a reuse distance and an equation from which the miss rate of the application may be solved;

FIG. 6 is a reuse histogram for two applications based on data that may be captured using sparse information about the reuse distance of Memory Accesses;

FIG. 7 is a diagram of a stream of Memory Accesses illustrating how the shape of a reuse histogram can be used to estimate a method for calculating the application's stack distance;

FIG. 8 is a flow chart of a method for randomly selecting one sample using a combination of hardware traps and single stepping;

FIG. 9 is a flow chart of a method for an alternative to the single-stepping techniques of FIG. 8;

FIG. 10 is a flow chart of a method for simultaneously detecting reuse distances for Memory Objects of different sizes;

FIG. 11 is a flow chart of a method collecting many samples and automatically adjusting the sample rate of the method;

FIG. 12 is a flow chart of a method estimating miss probability for a specific memory object based on the notion of how many replacements have occurred since this memory object was last touched;

FIG. 13 is a flow chart of a method estimating the amount of memory objects used by a programs sequence including of a target application based on data from sparse samples;

FIG. 14 is a flow chart of a method estimating miss rate for a fully associative cache with random replacement based on data from sparse sampling;

FIG. 15 is a flow chart of a method building a translation table to convert reuse distance data into stack distance data;

FIG. 16 is a flow chart of a method compensating the miss rate estimation on FIG. 14 for limited associativity cache implementations;

FIG. 17 is a flow chart of a method estimating miss rate for a LRU taking limited associativity into account;

FIG. 18 is a block diagram of a computer system;

FIG. 19 is a flow chart of a method gathering memory usage information for a program of instructions; and

FIG. 20 is a flow chart of a method estimating the number of unique memory objects in a segment of a program.

DESCRIPTION OF A PREFERRED EMBODIMENT

The ensuing description provides exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the ensuing description of the exemplary embodiments will provide those skilled in the art with an enabling description for implementing an example embodiment of the invention. It should be understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention.

System Overview

FIG. 1 is a diagram of a system according to an embodiment of the invention for measuring of a specimen or target software or other code to be analyzed. Host Computer System 110 may be used to execute a sequence of instructions in response to a program, a group of programs or an entire system, that is the target of the analysis, here designated as Studied Application 120, for which memory usage information should be captured. The memory usage information is gathered by a sampler method, represented as Sampler Mechanism 130, and collected as Application Characteristic Data (ACD) 140. The sample method includes selective monitoring of system parameters which can be used to estimate cache usage, cache hit/miss ratio, overall software execution time, etc. ACD may for example be stored as a data file on the host Computer System 110 and may be used as input to the System Model 150, which can model the behavior of a system, such as Host Computer System 110 or other computer systems. System Model 150 determines the characteristics of the Studied Application with respect to some modeled computer system. The characteristics can be provided for display using a suitable Graphical User Interface, 160, as depicted in FIG. 1, or any other means of transferring such information. The characteristics can also be fed directly to the runtime system of Studied Application 120 to allow for on-line and/or real-time adjustments; to the operating system of Host Computer System 110: to the compiler used to produce the binary of Studied Application 120 (e.g., machine executable object code); to a virtualization layer that may be controlling system resources; or, to any other piece of the system used to produce and manage the Studied Application or any aspect of the Host Computer System it is running on. In one embodiment of the described system, the gathering of memory usage information ACD is performed at runtime while system modeling may be performed as an off-line activity.

Gathering Memory Usage Information

Information of memory usage of Studied Application 120 (i.e., ACD 140) is gathered by a Sampler Mechanism that monitors various system parameters including, but not limited to, for example, properties of Memory Access, etc. The activity of such a Sampler Mechanism is shown in FIG. 2. Host Computer System 110 includes at least one Central Processing Unit, CPU 210 and a Memory System 220. Memory System 220 of the Host Computer System 110 may include one or more data and instruction caches as shown in further detail in FIG. 3. Studied Application 120, for which the ACD is to be captured, is assumed to be currently executing on or under the direction of CPU 210 and continuously producing a stream of Memory Accesses 230 to Memory System 220). For ease of illustration and explanation, in the present example, nine Memory access are shown, each representing either a read or write access to one of five different Memory Objects of Memory System 210, referenced to as A-E. A Memory Object (i.e., data referenced for retrieval or storage by an operation of memory access type) may be of a fixed size, for example corresponding to the cache line size, or may be of different sizes.

While it would be possible to collect information about every Memory access in the ACD, such a Sampling Method would be very slow and produce a vast amount of data in ACD. A faster method according to embodiments of the present invention is to only collect ACD from a relatively small subset of the accesses to Memory System 220, the data from this sampling method herein references as Sparse Application Characteristic Data. The terms Sparse Application Characteristic Data and ACD used in this document can mean both full ACD information as well as sparse ACD information.

Such sparse sampling is shown for the Memory Access of FIG. 2, where the Sampling Mechanism is configured to sample Memory Access number 1 and 5 (e.g., accesses made during sampling events 1 and 2). Since only a sparse subset of Memory Access is monitored, a substantial amount of memory usage data may be gathered for each Memory Access studied, here called Sample. In one embodiment, the Sparse Application Characteristic Data is collected in completely or effectively random or pseudorandom way. If the Samples are selected in such a way that Memory Access produced by the execution event of one instruction are sampled with a higher probability than Memory Access produced by the execution event of another instruction, some types of conclusions drawn by the System Model may not be representative of actual program execution. Other embodiments include Sampling systems with a more biased selection of operations to be monitored.

One parameter targeted by embodiments of the present invention is Reuse Distance as computed based on sampled Memory Accesses of each sample, i.e., the duration between the sampled Memory Access and the next Memory Access directed to the same Memory Object in the Memory System. It should be noted that many different time units would be possible for implementing the Methods described herein for purposes of the present illustration: including but not limited to the number of instructions; wall-clock time (e.g., real-time); or system time. In one preferred embodiment, duration is measured by counting the number of intervening memory instruction.

For example, referring to FIG. 2 and Memory Access stream 230, the first and the fifth access are randomly or otherwise selected to be sampled. For the first of these Samples the Reuse Distance is five, since there are five Memory Accesses between the sampled access and the next Memory Access touching the same Memory Object (object A at time 1 and next at time 7). The Sparse ACD 240, shown to be recorded in FIG. 2, is but one example of information that may be collected in the ACD for each of the two samples including a timestamp for the second access to Memory Object A (T=7), the address accessed during the first access to Memory Object A (D1=0x4564), the address accessed the second time Memory Object A was referenced (D2=0x4568), the Reuse Distance between the Memory Access (D=5), the address of the first instruction accessing Memory Object A (I1=0x0004), and the address of the second instruction accessing Memory Object A (I2=0x001C). The corresponding information is also shown for the second Sample with reference to Memory Object B (T=9, D1=0x1234, D2=0x1234, D=3, I1=0x0014, I2=0x0024). Examples of other types of information that mat be collected, include but are not limited to, I1+: the address of the next Memory Access instruction following I1, I1−: the address of the Memory Access instruction preceding I1, CID: the identity of the current context, and CS: the call stack information with hierarchical information about the instructions from which calls were made which led to the invocation of the current function/procedure for each of the instruction addresses of the sample, etc. It should be noted that the type of information collected shown for each sample is just one of many possible combination of information that may be collected in an ACD.

Sampling Techniques

There are several options for the implementation of Sparse Sampling Methods. One possible realization is based on a hardware-based implementation that randomly collects samples and records the corresponding ACD information.

Another implementation uses an instrumentation-based approach based on code instrumentation, where some of the Studied Application's instructions that accesses memory has been expanded into several instructions which add the additional functionality to perform the sample selection as well as recording the necessary ACD information for each sample. Examples of such rewriting techniques include, but are not limited to, static rewriting techniques and a dynamic rewriting technique. One technique for such an instrumentation-based sampling selection approach may be implemented as a counter which is initialized to the number of memory instructions remaining until the next sample should be taken. The counter is decremented each time a new memory instruction is executed, after which the counter's value is checked to determine if it is time to take a sample. Taking a sample mechanism may include the steps of recording of the necessary ACD data and enabling a watchpoint mechanism for some Memory Objects in such a way that the next access to the memory objects will cause further actions of the sampling mechanism and possible collection of more ACD data.

Collection of ACD data may require special processing considerations. For example, measuring the reuse distance of a sample may require recording the corresponding data address and comparing each following memory instruction with this value in order to determine if the same data has been referenced again. If so, the number of times the counter has been incremented since the corresponding sample was initiated will determine its reuse distance. Someone skilled in the art realizes that there are many efficient alternatives for implementation of the comparison mechanisms searching the Memory Object addresses of all active watchpoint, including some hashing techniques.

Instead of explicitly implementing the watchpoint mechanism, one can use some externally provided trap mechanisms. The externally provided trap mechanism may be in the form of software, such as a shell or operating system, and/or hardware, e.g., a breakpoint register and related logic.

Still another method to collect ACD uses a trap-based implementation, relying on hardware counters and trap mechanisms often present in modern CPUs to implement the sampling selection as well as recording of ACD associated with the samples. Modern CPUs often have hardware counters configured or configurable to monitor various events, such as the number of memory instructions executed or the number of CPU stall cycles that have occurred. Each time such a counter reaches some maximum value or otherwise reaches some predetermined threshold, an overflow trap is generated. Such a memory instruction counter can be initiated in such a way that a trap is generated exactly when a new sample is to be taken. This method allows the Sample Mechanism 130 in FIG. 2 to be “woken up” in response to a counter-initiated trap event using such a software trap handler. In response to the counter-initiated trap event, the trap handler can determine access information, such as the address of the Memory Object being referenced or “touched” by sampled memory instruction, the address of the instruction performing the access and the point in time the attempted access was initiated or occurred.

The watchpoint mechanism can also be enabled as a so-called Watch Point for the Memory Object containing the accessed data. A Watch Point is a mechanism often supported by an operating system, and may be supported and/or implemented in hardware. Enabling a Watch Point will ensure a software trap is generated the next time that Memory Object with a specified address and size is touched. This way, Sample Mechanism 130 of FIG. 2 will also be woken (e.g., activated) the next time that Memory Object is touched. At such time the handler can determine the access information, such as address of the instruction making this second access to the Memory Object, and the address of the part of the Memory Object being touched. The reuse distance between the sample event and this next access to the Memory Object may also be determined by assessing the value of a hardware counter counting the number of memory instruction when the Watch Point was initiated as well as when the Watch Point trap occurred. Many alternative ways for enabling watchpoint mechanisms to detect the next access to a Memory Object, similar to a Watch Point, exist. One possible way would be to alter the representation of the data such that an access to it could be recognized. Examples of this include setting a recognizable signature in the EEC code associated with the Memory Object or writing some value to some other bits associated with the Memory Object which could cause a trap or be recognized by any other means by some future access.

Compensating for Skid

The exact functionality of the hardware counters and the trap mechanism available in many processors sometimes requires more elaborate methods to perform the trap-based sampling. One problem that arises is the skid property of the overflow trap, i.e., the trap may occur several cycles after the instruction generating the counter overflow. For example, an interrupt routine responsive to an overflow condition may be delayed so that subsequent interrupt processing is not timely. This is referred to as an imprecise hardware trap. To further complicate the picture, skidding often has a property that makes it more likely to stop on certain instructions than others after such an event. So, using the overflow trapping mechanism would violate the randomness property used by the modeling method according to preferred embodiments of the present invention.

One method to overcome the skidding problem is to initialize a counter (hardware or software implemented) to generate a corresponding trap several memory cycles in advance of when a target sample is to be taken, i.e., trap “too early”. Turning again to FIG. 2, the sampling mechanism intending to sample instruction 5 would instead initialize the hardware counter to generate a trap for the 2^(nd) Memory Access of Memory Access stream 230. The skid phenomenon might result in the next (3^(rd)) instruction executing before the trap is generated and the sampling mechanism can be activated, but prior to the targeted fifth instruction. The sampling mechanism would read the hardware counter to determine that the sample should be taken exactly two memory instruction further down in the memory instruction stream (5−3=2). The sample mechanism would thereupon implement a more careful, but also may be more costly, method than the counter overflow trap, such as instruction single-stepping, to advance to the chosen 5^(th) instruction of the instruction stream.

A more detailed example of one possible way to handle the skid problem can be found in FIG. 8.

At step 810, the number of remaining Memory Access X before a sample is to be collected is determined. One possible way to determine X is to generate a random number between one and twice an expected average sample rate (the average ratio between monitored operations and the total number of operations), which yields a uniformly distributed random value between one and twice the designated sample rate. All other known distributions functions, such as Poison and exponential distributions, may also be candidates in this step. Furthermore, it is sometimes preferable to vary the sample rate over time, for example collecting several samples close in time to form a Sample Batch, and to place the Sample Batches further in time. Such consideration may also be incorporated in this step.

Step 820 shows one possible way to calculate the number of Memory Access that can safely be executed before a trap event occurs, while still guaranteeing that the corresponding trap will halt the execution of the Studied Application before the memory instruction that step 810 decided to sample, and to enable such a trap mechanism. The trap event may be based on a memory operation counter that generates an overflow trap when it reaches its maximum value, or some other available mechanisms. In one embodiment, a different kind of counter, such as an instruction counter, memory read counter or timer interrupt, may be used. It should be noted that many more sample time units than the number of Memory Access would be possible for implementing the Methods described herein for purposes of the present illustration: including but not limited to the number of instructions; wall-clock time (e.g., real-time); or system time. Examples in accordance with preferred embodiments of the present invention where the unit corresponding to the number of Memory Accesses is used, such as where the reuse distance is estimated, other kinds of suitable time units or equivalent temporal or sequence indicators may also be used, such as the time units listed above.

At step 830 the execution of the Studied Application is resumed in a “run” mode of operation wherein the Studied Application is executed normally.

Step 840 occurs at the point in time when the trap event has occurred plus the time the skid has added. In one embodiment of this step, the execution of the Studied Application is halted and a trap handler has been activated and will perform step 880, which is an alternative way of advancing the execution in a more controlled manner which may be realized as sub-steps 850 through 870.

At step 850 it is determined how many more operations should be performed before a sample is collected. This could be done by reading the memory operation counter to determine exactly how many more memory operations should be executed before the sample is taken. Another possible embodiment would be to calculate a random number according to some distribution function to randomly decide how many more memory operations should occur before a sample is taken.

At step 860 the execution is advanced by some known quanta. One possible implementation would be to advance the execution one instruction, while another possibility would be to advance the execution one memory operation. Other options include, but are not limited to, advancing several instructions or memory operations.

At step 870 it is determined if the correct (e.g., desired) number of memory operations have been executed. If not, some means of executing more memory instructions is invoked. One possible way of executing more instructions is to go back to step 860 again along the “yes” leg. If step 870 determines that the right number of instructions has been executed, the execution is halted on the memory operation to sample. Another possible embodiment would be to perform the sampling at this point without stopping the execution.

Multistepping

In one possible embodiment, step 880 is realized by the steps shown by the flow diagram of FIG. 9 illustrating a particularly efficient method for exactly advancing to the chosen memory operation.

At step 910 it is determined how many more operations of a certain type should be performed before a sample is collected. This may be implemented using similar techniques to step 850 of FIG. 8. The number of remaining operations is referred to as R.

At step 920 the instructions of the current basic block are inspected. This may be done by a disassembly function where the identity of each instruction in the basic block is decoded. At this step the number of operation of different type of the current basic block may be counted.

At step 930 it is determined if the current basic block contains R or more memory operations. If so, step 931 is the next step, as show by the right leg of 930.

At step 931 the R:th operation of the designated type in the current basic block is marked in a certain way. One possible way to do this is to set a breakpoint for that operation, another possible way would be to alter the representation of this instruction such that the operation could be recognized the next time it is executed.

At step 932 the execution of the Studied Application is resumed.

At step 933 the previously marked instruction is executed and recognized in a way that wakes up the Sampler Mechanism. In one possible embodiment, the execution is halted on this operation to allow for a Sample to be taken. Another possible embodiment would be to perform the sampling at this point without stopping the execution.

If step 930 determines that there is not R or more operations of the designated type remaining in the current basic block, 940 is the next step, as shown by the “no” leg of 930.

At step 940 the number of remaining operations of the designated type is reduced by the number operations of that type that remain in the basic block.

At step 950 the last operation of the current basic block is marked in a certain way. One possible way to do this is to set a breakpoint for that operation, another possible way would be to alter the representation of this instruction such that the operation could be recognized the next time it is executed

At step 960 the execution of the Studied Application is resumed.

At step 970 the previously marked instruction is executed and recognized in a way that they wake up the Sampler.

At step 980 the next operation to be performed following the last operation of current basic block is determined. This may be done by interpreting the execution of this operation. Another embodiment would access this information directly from hardware, such as reading the “next program counter” register or some similar information. Yet another possibility include single-stepping this very instruction and then determining the address of the next instruction. Further options include disassembling the branch instruction and determining the outcome of the branch based on register and/or stack content.

At step 990 the current basic block is set to the basic block containing the next operation determined by step 980, after which step 920 is performed again.

Sampling Memory Objects of Different Sizes

Some memory system models require that samples corresponding to Memory Object of several different sizes be sampled simultaneously. It may even be possible that one sample requires watchpoint mechanisms to be enabled in such a way that several Memory Object sizes are monitored. This could, for example, be required for modeling a memory system as shown in FIG. 3, if different block sizes are used, e.g., if the block size of the first level data cache 330 is 16 bytes and the block size of the second level cache 340 is 64 bytes. There are also for example situations where the explicit data object size corresponding to the size of the data read or written by the sampled memory operation should or needs to be monitored, such as monitoring accesses to one word in memory. This necessitates that ACD be collected for Memory Objects corresponding to both sizes on the Host Computer System.

One efficient way to collect ACD for Memory Objects of different sizes is to first set up the Sampler Mechanism to monitor the reuse for the largest object size, in the case of the present illustrative example, 64 bytes. Once the next reuse to this larger object has been detected it is determined if this to the largest object also accessed the smaller object (i.e., the 16 byte object). If so, the reuse of the smaller Memory Object is identical to the reuse of the larger object. If not, the sampling mechanism is configured to determine the reuse from this point in time to the next time the next smaller Memory Object is touched and calculate its reuse distance as the sum of this distance and the reuse distance of the larger object. There are several advantages to this approach over the technique of setting up a Watch Point for each memory object size. This approach only requires one active Watch Point for each sample regardless of the number of different Memory Object sizes. Furthermore, the spatial locality often present in the access pattern of a Studied Application will cause the watchpoint mechanism for a large Memory Object to often also detect reuse for several of the smaller Memory Object sizes.

FIG. 10 is a flowchart for an efficient method of monitoring Memory Object of several sizes.

As step 1010 the appropriate sample is selected and necessary ACD data, such as the address of the instruction address of the sample, the address being accessed by it and a timestamp of when the sample occurred, is recorded. It should be noted that once this step has completed the collection of ACD data for this sample has been started. Meanwhile new sampling activities for other Samples may be started in parallel with the remaining steps for this Sample, marked as sampling has started in FIG. 10. It should be noted that the exact point in time (or other suitable demarcation) after which other sampling activities may be started vary between different implementations. For example, the arrow corresponding to a start of sampling may originate right after box 1050.

As step 1020 the accessed address A of the accessed Memory Object and the number of memory operations performed (T) is recorded. In one possible embodiment this information is retrieved by reading special hardware counters and registers. Furthermore, the largest Memory Object size to be sampled is recorded as S.

At step 1030 a Memory Object of size S containing the address A is selected. One possible way to apply the same division of the memory address space into Memory Objects as is used by the memory system to model, such as hashing on some specific address bits, but other ways of defining Memory Object boundaries are also possible.

At step 1040 the Memory Object selected is marked in a certain way. One possible way to do this is to set a Watch Point for a Memory Object of the selected address and size, another possible way would be to alter the representation of the data such that the an access to it could be recognized.

At step 1050 the execution of the Studied Application is resumed.

At step 1060 the previously marked Memory Object is accessed by an access to address B, which is recognized in a way that wakes up the Sampler Mechanism. In one possible embodiment, the execution is halted on this operation to allow for a ACD data to be recorded by the Sampler Mechanism. Another possible embodiment would be to collect this data at this point without stopping the execution.

At step 1070 the reuse distance of this sample for the studied Memory Object size S is recorded. One possible way of doing this is to determine the current number of memory operations performed and to subtract the recorded value T. At this step necessary ACD data, such as I2 address, D2 address, the reuse distance, etc, may be recorded,

At step 1075 it is determined if ACD data for all studied Memory Object sizes have been recorded for this Sample. If so, the activity for this sample is terminated as shown by the “no” leg of 1075, else we turn to step 1080 as shown by the “yes” leg of 1075.

At step 1080 the next largest Memory Object size is set to S.

At step 1090, a Memory Object O of size S containing the address A is selected similarly to step 1030.

At step 1091 it is determined if the new Memory Object O contains the address B, if so we turn to step 1070 to record the reuse distance for this Sample as shown by the “yes” leg, else we turn to step 1040 as shown by the “no” leg to set up a Watch Point for Memory Object O with size S.

Automatic Adjustment of Sample Rate

Short-running programs typically require a lower sample rate (i.e., will need to sample more often) that long-running applications. A sample method which dynamically adjusts the sample-rate would be beneficial. For example, the sample rate can be adjusted such that samples are taken less often (i.e., less frequently) for long-running applications. One possible embodiment of such a method is described with reference to the flowchart of FIG. 11.

At step 1110 the preferred number of samples P to be collect during the execution is determined. This may be done by predefining the value of P or by making the value of P configurable. In one possible embodiment the value of P predefined to 20.000.

At step 1120, the initial Sample Rate N is determined. This may be done by predefining the value of N or by making the value of N configurable. In one possible embodiment the value of N could be set to 100.000 resulting in one sample taken on average for every 100.000 Memory Access.

At step 1130 we start taking one Sample and record necessary ACD data. One possible way of doing this is according to the steps defined for 1001 in FIG. 10.

At step 1140 it is determined if we have taken a specified fraction of the preferred amount of samples using the current Sample Rate already. In one embodiment we could determine if at least half the preferred number of samples has been taken using the current Sample Rate. If so, we turn to step 1150 to adjust the Sample Rate, if not we turn to step 1130 as shown by the yes leg of 1140 to start to take yet another sample, else the “no” leg of 1140 will lead to step 1150.

At step 1150 the Sample Rate is adjusted. In one embodiment the rate N will be doubled to cause samples to be taken half as often on average.

Turning now to FIG. 19, describing steps for gathering memory usage information for a program of instructions. As a first step 1910, the execution of the program from which the memory usage information should be gathered is started. This will cause a sequence of operations, such as CPU instructions, database accesses, accesses to disk, or any other possible operations, to be initiated.

As a next step 1920, an operation to be monitored is chosen. This may be done according to a random selection scheme (e.g., uniformly distribution selection), or as a more biased selection (e.g., the next time an operation of a specific type, from a specific function is executed or an operation touching memory objects from a specific address range), or any other selection scheme.

Step 1930 may record specific information associated with the selected operation, such as the address of the memory objects it is accessing, instruction type of the selected operation. Another kind of information than may be recorded is call stack information. Call stack information identifies the history of functions, procedures, methods, tasks and similar program constructs used as a conduit to reach the operation. In one embodiment, the call stack information consists of the sequence of return addresses present in the activation records on the stack. In another embodiment, the call stack information contains the addresses of function entry points. In yet another embodiment, the call stack is augmented with function call parameter values. The call stack may be complete and leading up to the program's or thread's initial entry point, or it may be truncated so that only a few levels of call stack below the top frame are included. At this step the identity of an of memory objects accessed by the operator is identified.

At step 1940 some kind of watchpoint mechanism is enabled. The watchpoint mechanism will detect the next time an operation accesses the identified memory object.

Step 1950 is triggered the next time an operation accesses the identified memory object. That operation is sometimes referred to as the next operation. During this step, specific information about this operation is recorded, for example the address of the memory objects it is accessing, instruction type of the selected operation. Another kind of information than may be recorded is call stack information.

Modeling a Memory System

FIG. 3 shows an example of a conventional Memory System 310 including separate first level (L1) caches for data (330) and instructions (350), a unified second level (L2) cache storing both instructions and data (340) and main memory (360). Also shown is the mass storage or persistent memory here represented by a disk system (370) and the Translation Lookaside Buffer (TLB) (390) which caches address translations from virtual to physical memory and quickly can determine the physical location in main memory which has been allocated to store a page P. Note that the system described here is just an example of possible systems. An ample number of possible combinations of different topology and cache types exists.

Modeling of such a system may be performed using ACD or Sparse ACD collected on a Host Computer System running the Studied Application even though the Memory System to model may be very different from the memory system of the Host Computer System where the ACD 230 was collected. The goal is to estimate the behavior of the Model System 310 as if it was executing the Stream of Memory Access 230 of the Host Computer System. One possible functionality of such a system could be the following: Each time a new access from the Stream of Memory Access 230 attempts to access a specific Memory Object from the memory system, the contents of the caches in the hierarchy are first searched for that Memory Object. First the smallest and fastest cache at the L1 level is searched, secondly, the second level of caches are searched and so forth. Only if the data cannot be found at any level of caches will the slow main memory need to be accessed.

Modeling the memory system involves algorithms for determining if the Memory Object where the instruction being executed resides, and in case of a Memory Access also determining if the Memory Object accessed by the instruction can be found in a cache, and if so, in which level of cache they can be found. The algorithms and techniques described in this disclosure according to various embodiments of the invention are not limited to caches of memory system, but can also be applied to any other systems implementing similar kinds of caching systems including, but not limited to, disk caching, web caching and the caching performed by the virtual memory system.

Cache Algorithms

In one method of modeling a Memory System, such as 310, the model chosen for each level of cache in the memory hierarchy should closely correspond to the kind of cache at that level. Most importantly, the size of the modeled cache must be appropriate. Further, caches can be classified according to the size of Memory Objects handled by the cache, called its block size (sometime referred to a cache line size); in how many optional places a specific Memory Object can be placed, called its associativity; and, its strategy for picking a victim among the optional places where a new Memory Object can be placed, for example random replacement (RAND), round-robin (RR), least recently used (LRU), pseudo LRU (PLRU) and not most recently used (NMRU) algorithms.

Another method of modeling a Memory System, such as 310, the model chosen would instead model an idealized system. Here, the purpose of the modeling is to study some specific property of the studied application. Examples of such systems include but are not limited to systems with infinitely large caches, with fully associative caches or with cache where the Memory Object size is equal to the data size accessed by each Memory Access.

Modeling a Fully Associative Cache

A fully associative cache 410 is shown in FIG. 4. A fully associative cache has the property that a specific Memory Object may be stored in any of its L location dedicated for storing Memory Objects. The Memory Object A (420) is depicted as placed in one such places. Assuming that Memory Object always gets installed in the cache in the case of a cache miss, we can conclude that the Memory Object A would reside in the cache with the probability 1.0 right after it has been touched. Further, assuming a random replacement (RAND) the probability that A resides in the cache after one cache replacement has occurred is (1−1/L). Subsequently, if n Replacements have occurred since A was last touched the likelihood of finding A in the cache can be calculated by the function P_hit 430 in FIG. 4, and the probability of not finding A in the cache can be calculated by function P_miss 440.

FIG. 12 shows a flow chart of some possible steps to determine the probability of a miss in a cache.

At step 1210 the number of locations L of the cache is determined. One possible way to determine this is to divide the cache size with the block size of this cache corresponding to the Memory Object size.

At step 1220 the number of replacements performed since a specific Memory Object A was last touched is determined. This may be done through counting of such events or by estimating the number of such events. In one embodiment, the number of replacements can be estimated by multiplying the assumed miss rate R with the reuse distance of the studied Memory Object A.

At step 1230 the probability P_hit(1), which is the probability that a given Memory Object resides in the cache after exactly one replacement, is calculated. In one embodiment, this is done using the formula P_hit(1)=(1−1/L), i.e., the value one subtracted by the result of a division between the value one and L

At step 1240 the probability P_hit(n), which is the probability that a given Memory Object resides in the cache after n replacement, is calculated. In one embodiment, this is done using the formula P_hit(n)=(1−1/L)^(n), where n is the estimated number of replacement performed.

At step 1250 the probability P_miss(1), which is the probability that a given Memory Object will not resides in the cache after n replacement, is calculated. In one embodiment, this is done using the formula P_miss(n)=1−(1−1/L)^(n), where n is the estimated number of replacements performed.

Turning now to FIG. 5, the memory instructions from the memory stream 230 can be found in the sequence at the top. If we assume that the miss rate is R, i.e., the likelihood that a memory instruction will not find its requested Memory Object in the cache is R, we can estimate the number of replacement that have occurred between the first and second access to the Memory Object to be its reuse distance multiplied by the assumed miss rate R. For the first sampled access of 230, 5R replacements are estimated to occur between the first and the second access to the Memory Object A and for the second selected sample 3R replacement will occur between the two accesses to the Memory Object B. Thus, the likelihood of a miss can be estimated by estimating P_miss by performing the steps described in FIG. 12. For the two examples in FIG. 5 such estimation is indicated by boxes 510 and 520. However, the P_miss equation only holds for misses caused by replacement activity, here called Capacity Misses. The likelihood of a cache miss is equal to 1.0 the first time a Memory Object with a size equal to the block size of the modeled cache is touched. One example of such a Cold Miss is Memory Access number 2 of 230, which is the first access to the Memory Object B.

Estimating Cold Misses and Working Set Size

The name Cold Misses is used to refer to cache misses that occur because the data is touched (e.g., accessed or addressed) for the first time. FIG. 13 is a flowchart of a method for estimating the amount of memory objects used by a program's sequence including, the cold miss rate and the working set for a fraction of execution of a Studied Application. The strategy outlined in FIG. 13 is to detect the last time a Memory Object is touched, rather that the first time it is touched. This is a novel way of estimating cold misses.

At step 1310 Application Characteristic Data is gathered including information about enabled watchpoint mechanisms for which no reuse distance (i.e., no next operation) have been recorded. The Application Characteristics Data collected at this step may be sparse. In one embodiment, this is done by performing the steps 1101 of FIG. 11. In another embodiment, the number of samples for which no reuse distance has been recorded is estimated to be the number of dangling Watch Points of step 1070, which is the number of enabled watchpoint mechanisms for which no next operation has been detected. Another possible alternative is to read previously recorded Application Characteristic Data including information about samples for which enabled watchpoint mechanisms no reuse distance have been recorded. In one embodiment, only Characteristic Data associated with a Memory Object size equal to the block size B of a modeled cache are being considered. It is further possible that only a certain selection of Characteristic Data, such as for example data from one segment of the execution, data associated with certain Memory Objects or data associated with certain instructions are collected at this step.

At step 1320 it is determined how many of the initiated samples' watchpoint mechanisms that still have not detected a next access to the corresponding Memory Object of the studied Memory Object size O. In other words, the fraction of enabled watchpoints for which no next operations has been identified is measured. In one embodiment, this involves determining how many samples that have not recorded a reuse distance value for the selected Memory Object size according to step 1070 of FIG. 10.

At step 1330 the fraction number produced by step 320 is multiplied by the total number of operations executed by the program.

In one embodiment, the number of samples collected at step 1310 is from one segment of the execution of the Studied Application why the calculations performed in the following steps result in estimations of corresponding parameters for that segment only.

Estimating Capacity Misses

The total number of capacity misses caused by replacement as discussed in FIG. 12, can be calculated by a summation over the total number of Memory Accesses not experiencing cold misses.

The total number of cache misses may be calculated in two different ways: first, the total number of Memory Accesses N is multiplied by the estimated miss rate R, secondly the number of cold misses plus the miss probability of all Memory Objects touched at least a second time. The likelihood that a Memory Objects touched at least a second time will experience a miss may be estimated by using the P_miss steps of FIG. 12, using its distance d(i) multiplied with the estimated miss rate R as the number of replacements. This results in the equation for the total number of misses:

$\begin{matrix} {{Tot\_ misses} = {{R \cdot N} = {{Cold\_ misses} + {\sum\limits_{i = 1}^{N}{{P\_ miss}\left( {{d(i)}*R} \right)}}}}} & \left\lbrack {{Equation}\mspace{14mu} 530} \right\rbrack \end{matrix}$

Someone skilled in the art realizes that if the total number of Memory Accesses N, as well as the number of Cold misses K, as well as a reuse distance d(i) for the N Memory Accesses, we can solve the value of R from Equation 530 numerically. It should be noted that d(i) is defined as zero if no reuse distance has been recorded and that P_miss(0) is zero according to the definition of P_miss(n).

A rewritten version of the equation, as defined by Equation 540, only estimates P_miss for samples with a recorded reuse distance, and thus does not require a definition of d(i) if no reuse has been detected.

$\begin{matrix} {{Tot\_ misses} = {{R \cdot N} = {{Cold\_ misses} + {\sum\limits_{i = 1}^{N - {Cold\_ misses}}{{P\_ miss}{\left( {{d(i)}*R} \right).}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 540} \right\rbrack \end{matrix}$

Equations 530 and 540 are both valid also for sparsely sampled ACD data where N is the number of samples and Cold_misses is the number of those samples which are estimated to be cold misses.

Each unique Memory Object used by an application will cause exactly one cold miss, why the Cold_miss variable in equations 530 and 540 may be replaced by Number_of_Memory_Objects.

Using Histograms

Furthermore, if we only know the reuse distance d(i) for a small but representative subset M of the Memory Accesses, equation 530 still hold for those M references and will result in a representative estimation of the R value. Here, the letter N is replaced with the letter M, Tot_misses is the estimated number of the M Memory Accesses that misses, the Cold misses is the estimated number of the M Memory Accesses that cause a cold miss and the summation goes from 1 to M. To further convince ourselves of this thesis, FIG. 6 shows the reuse histogram 610 for two Studied Applications. The value h(d) on the Y-axis for each X-value of d shows the number of Memory Accesses recorded with exactly that value. Each application shows the shape of their corresponding reuse histogram based on every access as well as based on the sparse sampling of every 10,000^(th) access (but scaled up). Such a histogram is one way of presenting the reuse distance distribution for a program, or a segment of a program. If the sparse sampling is done in a random and completely independent way, it should come as no surprise that the sampled shape corresponds well with the shape based on every access. Multiplying each h(d) value for a histogram build from sparsely sampled ACD with its Sample Rate results in a new histogram equivalent with histogram based on non-sparse data.

Since equation 530 can be transformed to the equation 620 based on histogram data it can easily be understood that sparse sampling can be used for solving the value of R.

Estimating Cache Misses for a Random Replacement Cache

FIG. 14 is a flow chart of possible steps for estimating the miss rate of a cache with respect to a sequence of accesses to a memory system.

Steps 1440 through 1490 are all one example of an iterative solver. Many alternative ways of alternative solvers exist and they are all possible embodiments for steps 1440 through 1490.

At step 1410 Application Characteristic Data is collected for example in a similar way to step 1310 of FIG. 13. This may be done by collecting reuse distance information for a selection of memory operations

At step 1420 a histogram based on the Characteristic Data collected may be built. In one embodiment, reuse distance property of the Characteristic Data is being targeted to make the entry h(d) of the histogram represent the number of Samples in the Characteristic Data that has a reuse distance equal to the value d. One may build a more coarse histograms where each entry h(d) represents the number of Samples for a range of reuse distances.

At step 1425 certain parameters of the target cache and values for the iterative solver are initiated. In one embodiment the number of locations L of the modeled cache is estimated by dividing the cache size S with the block size B. Possible iteration values initiated are a high guess value High, set to 1.0 and a low guess value Low, set to 0.0.

At step 1430 the number of cold misses is estimated. One way of doing this is to use the steps described in FIG. 13. In one embodiment, only Cold misses associated with the selected data of step 1410 are considered. Since the number of cold misses is equal to the number of unique Memory Objects, of the same size as the Objects in the cache, accessed by the sequence of accesses, the number of unique Memory Objects may be used instead of the number of cold misses.

At step 1440 a value of the miss rate R is estimated. This may be done by calculating the average between the high guess High and the low guess Low values.

At step 1450 the number of capacity misses is estimated. This may be done by summing up each histogram value h(d) multiplied by the corresponding Miss Probability 1201, where the number of replacement n is of step 1240 is estimated to be the value d multiplied by R. In an alternative embodiment, the miss probability for Samples selected in 1410 may be estimated individually and then summed together. The individual miss probability may be calculated using the Miss Probability steps in 1201 of FIG. 12, where the number of replacement n is of step 1240 is estimated to be the reuse distance d of the Sample multiplied by R.

At step 1460 the total number of misses caused by the samples selected in 1410 is estimated. This may be done by multiplying the number of samples selected with the estimated value of R.

At step 1470 a convergence value CONV is calculated. This may be done by setting the value CONV to the result of subtracting the estimated cold misses and subtracting the estimated capacity misses from the estimated total misses.

At step 1480 a convergence test is performed. In one embodiment, it is determined of the absolute value convergence value CONV is smaller than a threshold. If so, the estimated miss rate is R, if not we turn to step 1490.

At step 1490 new values for the estimate variables are estimated. In one embodiment, the value of the convergence test is assessed. If the convergence value CONV is larger than 0.0, the value High is set to R. If the convergence value CONV is smaller than 0.0, the value Low is set to R. Following step 1490, we turn again to step 1440 to perform a new estimation of the miss rate R.

Modeling an LRU Cache

A cache based on least recently used (LRU) replacement algorithm will require a different method for estimating its miss rate. Knowing the so-called stack distance for each Memory Accesses would here be a good help. While reuse distance is the total number of Memory Accesses between the two accesses to the same Memory Object, stack distance is the number of Memory Accesses to unique Memory Objects. If the stack distance is smaller than the number of Memory Objects that the cache can hold, called L for the cache 410 in FIG. 4, the second access to a Memory Object will result in a cache hit, or else a cache miss has been estimated.

In order to make use of the reuse distance data collected to also model LRU caches, a new method for conversion between reuse distance and stack distance is needed.

Turning now to FIG. 7, the sequence of Memory Accesses 230 have been augmented with the reuse distance arrows for each the first six Memory Accesses, named d(1) through d(6). By studying the different Memory Object accessed during the Reuse Area of A, defined as the Memory Accesses between the first and the second access to A, we can determine that the corresponding stack distance for this access is 3, since only the objects B, C and D are touched during this time period. Another way to draw this conclusion is to look at the reuse distance arrows for d(2) through d(6), and determine how many of these point beyond the Reuse Area. In this example d(4), d(5) and d(6) all point beyond the second access to A, this tells us that the stack between the two references to A is 3. This way of estimating the stack distance can be generalized for a Studied Application with a reuse histogram h(d). Starting with a histogram 730 in FIG. 7, we can calculate the function 740 P_(GTD)(X), showing the probability that a Memory Access of the application has a reuse distance which is larger than a certain value x, based on the summation of the reuse histogram values from i=0 to i=x−1 as shown by the formula 760. Determining the average stack distance S(D) 750 can now easily be done for a given value of D by summing up the probability that each of the D Memory Accesses in the Reuse Area is pointing beyond the Reuse Area, as shown by formula 770. We can now calculate the miss probability for each memory access i by determining if its stack distance S(d(i)) is larger or smaller than L. In one embodiment the reuse histogram of a Studied Application, which easily can be estimated by sparse sampling as shown in FIG. 6, can be used as input to estimate the stack distance S(d(i)) using the above method.

Estimating Stack Distance

FIG. 15 shows a flow chart of how to build a translation mechanism translating reuse distance to stack distance, this implies that for a given sequence of memory operations consisting of some number of memory operations FIG. 15 describes one method of estimating the number of unique memory objects touched during that segment.

At step 1510 Characteristic Data is collected similarly to step 1310 of FIG. 13. In one embodiment, this collection could be done by performing step 1101 of FIG. 11. Another way may be to read already saved ACD data from memory or from a file.

At step 1520 a histogram is built similar to step 1420 of FIG. 14.

At step 1530 the probability vector is P_(LTD) (probability that a sample has a reuse distance smaller than d). In one embodiment, this may be done by summing up all histogram values between h(0) and h(d−1) and dividing the result by the total number samples.

At step 1530 the probability vector is P_(GTD) (probability that a sample has a reuse distance greater than d). In one embodiment, this may be done by subtracting the corresponding value G_(LTD) from the value 1.0.

At step 1550 the actual translation table S(D) is put together. Each entry of S(D) is calculated by summing up all values between and including P_(GTD)(1) to P_(GTD)(D).

Given the translation table S(D), the stack distance for each Sample i can be estimated by performing a lookup SD using its distance rd(i): Stack_distance(i)=S(rd(i)). Let L total number of Memory Objects in the modeled cache. Then, an estimated stack distance smaller than L result in a hit estimate with the estimated probability of 1.0, while a stack distance larger than L result in a miss estimate for the Sample. This yields the equation for the total number of misses in an fully associative LRU cache:

${Tot\_ misses} = {{Cold\_ misses} + {\sum\limits_{i = 1}^{N}\left( {{{if}\left( {S\left( {{{rd}(i)} < L} \right)} \right)}\mspace{14mu}{then}\mspace{14mu} 1\mspace{14mu}{else}\mspace{14mu} 0} \right)}}$

where N is the number of selected Samples from ACD to use as input.

Turning now to FIG. 20, estimating the number of unique memory objects in a segment of a program of instructions. At step 2010, a representative reuse distance distribution for memory operations of the segment is collected. One way to do this is to create a histogram over the reuse distance recorded for all operations of the segment, or to estimate a histogram based on a selection of operations of the segment. A histogram may also be collected based on operations close to the segment or based on the operations of the entire program, if that will produce a representative distribution.

At step 2020, the number of memory operations performed during the segment is estimated to be D. This corresponds to the reuse distance of the segment.

At step 2030 the natural number series from one up to D defined in step 2030 is examined and for each such number, a probability fraction is calculated as the amount of objects with a larger reuse distance than that number in the histogram is divided by the total number of objects in the histogram.

As step 2040, the probabilities estimated in step 2030 are added together.

In summary, steps 2030 and step 2040 can be summarized by the equation:

${{number\_ of}{\_ unique}{\_ memory}{\_ objects}} = {\sum\limits_{i = 1}^{D}{P\left( {{reuse\_ dist} \geq i} \right)}}$

A More Accurate LRU Model

While the stack distance estimation described above will result in a good estimate for large reuse distance values, shorter reuse distances would need to include more detailed information or knowledge about the relationship between reuse distance and stack distance. Especially, the histogram bin that corresponds to the reuse distance minus one will require more attention. That particular reuse distance would correspond to a distance that coincides with the trigger instruction for the reuse distance under consideration. Since we know that none of the memory operation in the Reuse Area of FIG. 7 can have their reuse distance d(x) pointing to the second occurrence of A, this has to be reflected in the Equation.

Consider an example where the reuse distance between two Memory Accesses to A is zero. Clearly, there can be no other accesses between the two Memory Accesses to A and the stack distance should be defined to be 0. This yields that S(0)=0 Now, consider a situation with a reuse with distance of one, i.e., between the two consecutive Memory Accesses to A is one access to a different memory object X (i.e., {A, X, A}). Despite the acquired sample's reuse distance distribution, the access X cannot have reuse distance 0, since it would then collide with the trigger access corresponding to the second access to A. From this example, it is evident that in this scenario, X's reuse distance cannot be one, since it would then coincide with the second access to A. When we calculate the reuse distance probability, we therefore temporarily remove the contribution from samples with exactly this reuse distance. This yields that S(1)=1

Now consider a situation with a reuse with distance of two, i.e., there are two references X and Y within this interval (i.e., {A, X, Y, A}), and where the projected average stack distance is between one and two. Now, consider access X. The sample histogram offers some information on the distribution of its reuse distance. For instance, the probability that X's reuse distance equals zero is h(0)/N, where N is the total number of samples included in the histogram. Generally, the probability that the reuse distance is D is given by h(D)/N. However, if X's reuse distance is one, both X and Y must access the same memory object while the stack distance between the two references to A is one. For all other histogram values for X and Y are to different memory objects and the stack distance between the two references to A is two.

$\begin{matrix} {{S(2)} = {{1*{h(0)}\text{/}\left( {N - {h(1)}} \right)} + {2*\left( {1 - {{h(0)}\text{/}\left( {N - {h(1)}} \right)}} \right)}}} \\ {= {1 + 1 - {{h(0)}\text{/}\left( {N - {h(1)}} \right)}}} \\ {= {{S(1)} + 1 - {{h(0)}\text{/}\left( {N - {h(1)}} \right)}}} \end{matrix}$ Continuing this reasoning gives the general recursive formula:

${{S(r)} = {{S\left( {r - 1} \right)} + {\sum\limits_{k = 0}^{r - 2}{{h(k)}\text{/}\left( {{Nh}\left( {r - 1} \right)} \right)}}}},{{{where}\mspace{14mu}{S(0)}} = 0.}$

Adjusting for Variable Miss Rate

All System Modeling techniques discussed so far assume a uniform distribution of the memory instruction's properties. Example of such assumptions is the miss rate R of equations 530 and 620, as well as the property of the Reuse Area of FIG. 7. While the described methods will still be a fairly good approximation, adding adjustments for the variations can improve the results even further.

One possible way to overcome this is to split up the execution in many time windows and to apply each equation to one time window at a time. In one possible embodiment the reuse histogram of one time window is used to calculate the miss rate R for that time window. The miss rate of the entire application is then calculated based on the miss rate of each time window. One alternative to statically division into time windows is to have sliding time windows.

Compensating for Set Imbalance

FIG. 16 shows a flow chart describing how to compensate for set imbalance for a cache with random replacement.

At step 1610 Application Characteristic Data is collected similarly to step 1310 of FIG. 13. In one embodiment, this collection could be done by performing step 1101 of FIG. 11. Another way may be to read already saved ACD data from memory or from a file or from a different computer for example using a network connection.

At step 1620, the samples collected in 1610 are sorted according to the set-hash function of the modeled cache, i.e., the function used to select one specific cache set as a function of the memory address accessed, and the number of samples per set is determined.

At step 630 the Utilization Factor is calculated for each set. The Utilization Factor is 1.0 for a set which received the average number of samples. A Utilization Factor higher then 1.0 indicate that the set is overused. The Utilization Factor for a set may be calculated by multiplying the number of samples targeted to the set with the number of sets of the modeled cache and then dividing the result by the total number of samples.

At step 1640 the miss rate without any consideration of set imbalance is calculated, for example using the method 1401 described in FIG. 14.

At step 1650 the samples of the ACD collected at step 1610 are used one by one to estimate their individual miss probability, for example using the method 1201 described in FIG. 12, using L of the modeled cache divided by the Utilization factor as the L value of method 1201.

At step 1660 the average of the miss probabilities of step 1650 is calculated.

FIG. 17 describes a method to estimate the miss ratio of an LRU cache while taking set imbalance into consideration.

At step 1710 a translation table S(D), translating reuse distance ACD into stack distance values, is built. One possible way to do this is according to step 1501 of FIG. 15.

At step 1720 the Utilization Factor is determined for each set. In one embodiment, this is done in accordance with step 1670 of FIG. 16.

At step 1730 all the samples of the ACD samples collected at step 1710 is accessed one by one. If the lookup of the table S(D) using the reuse distance D of a sample results in a stack distance value is larger than the L value of the modeled cache divided by the Utilization Factor of the corresponding set, a miss probability of 1.0 is recorded for that set. Else, the miss probability of 0.0 is recorded.

At step 1740, the average miss probability estimated for each sample in step 1730 is calculated as an estimate of the average miss probability for the Studied Application.

Adjusting for Semi-Static Conflicts

All System Modeling techniques discussed so far are targeting caches organized as fully associative caches with the property that a Memory Object may be placed in any of their L locations. A more common organization for caches is the so-called set-associative organization, where each specific Memory Object can only be placed in the subset consisting of W locations of the L total locations in the cache. Such an organization is called W-way associative. Organizations ranging between 4-way and 16-way are common today. The collection of possible places where a Memory Object can be placed is called a Set. A part of the Memory Object's address is often used to determine which Set it belongs to. An N-way associative cache with L locations has L/W Sets, where W is the number of associative ways in each set of the cache We denote the number of sets in a cache S.

N-way associative caches result in a higher miss rate that fully associative in practice for two reasons: Memory Objects accessed by a program are not uniformly distributed over the sets and the replacement algorithm, such as random or LRU, can only make their pick among W Memory Objects rather than L object.

We can measure the Set Ratio describing how the Sampled Memory Object are spread over the different Sets during the execution of the Studied Application. With an even distribution of Memory Accesses, each set should be accessed with the Set Ratio 1/S. The reference to a set with the ratio 2/S will effectively experience a cache with only L/2 location using the formulas. Adjusting the cache size in such a way on a per-sample basis in the above mentioned methods will effectively take some of the negative effects of set-associative caches into account. If the Set Ratio is measured and their effect adjusted for Samples from shorter time windows during the execution, even some semi-dynamic effect of cache conflicts will be taken into account.

Adjusting for Frequent Reuse Areas

For some Studied Applications one specific Reuse Area measuring the reuse between the same two instructions I1 and I2 may be common (Reuse Area contains all intervening accesses between instruction I1 and I2). If so, special measures can be taken to study this Reuse Area extra carefully. By collecting statistics about the other instructions and Memory Objects sampled during the different occurrences of the Reuse Area, it can be determined if conflicts in the caches of different levels are likely to occur with the reuse Memory Object of the Reuse Area. If one occurrence of the specific Reuse Area is measuring the reuse distance for a Memory Object with the address A and three new Samples accessing different Memory Objects are taken during this Reuse Area, one of which accesses a Memory Object hashing to the same Set as A, we record a 33% conflict rate for this occurrence of the Reuse Area. Combining such information with those of other occurrences of the same Reuse Area allows for the conflict rate for the Reuse Area to be calculated. Through this technique it can be determined that a Reuse Area recording a reuse distance smaller than L still would yield a cache miss due to cache conflict. This way, also dynamic conflicts can be calculated for frequent Reuse Areas.

The usefulness of this technique will increase if samples are collected in Batches, i.e., time periods during which Samples are collected more frequently. The time period between the Batches can be rather long though, depending on the Sample Rate.

Modeling Instruction Misses

Even though the methods described so far describe how to estimate the miss rates of memory caches, such as cache 330 of FIG. 3, someone skilled in the art can easily use the described methods to also estimate miss rates of instruction caches (350) or unified caches (340). The reuse distance for Instruction Objects can be measured using so-called trace-points on the instruction addresses of the Instruction Objects.

Modeling Multiple Cache Levels

FIG. 3 shows an example of a modern memory system consisting of several cache levels. Assuming full inclusion between the different cache levels, modeling such a system using the above described methods is possible. Full inclusion between cache levels implies that if a Memory Object or Instruction Object resides in the higher-level and larger caches, they should also reside in the corresponding lower-level caches. With that assumption, the number of misses can be estimated independently for the L1 instruction cache 350 and the L1 data cache 330 using separate reuse histograms for instructions and data with appropriate block sizes. In order to estimate the number of misses for the unified L2 cache a joint histogram with instructions and data with the block size of the L2 cache needs to be merged from the separate instruction and data histograms. If the technique with time windows is used for the histograms, such a merge is needed for each time window.

Handling Samples from Different Contexts

The CPU executing the Studied Application may perform a context switch and start executing a different context, such as a different process and/or a different thread, fiber, strand, task or co-routine. In order to handle this case correctly, a virtualization of the hardware counters may seem necessary, i.e., the value of the hardware counters should be saved and restored by the operating system, or in some other way, such that it appears that each context has its own set of hardware counters, watchpoint mechanisms, trap mechanisms and/or Watch Points.

If the system running is lacking virtualization of the hardware counters, this may be compensated for be the samples mechanism and/or the system model methods. One way of doing this is to tag a context ID (CID) to each of sample and to determine if some other context has been sampled between the sample was initiated and the reuse distance was recorded, i.e., during the Reuse Area as shown in FIG. 7. If so, the fraction of samples started by this context may be determined by dividing the number of samples started by this context with the total number of samples started during the Reuse Area. The reuse distance recorded may then be adjusted by multiplying its value with this fraction.

Alternative Implementations

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. It should be noted and understood that certain publications, patents and patent applications mentioned in this specification are indicative of the level of skill in the art to which the invention pertains. All publications, patents, and patent applications are herein incorporated by reference to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety. 

What is claimed is:
 1. A method of gathering memory usage information for a program of instructions, the method, executed by a processor and including the steps of: starting to execute a sequence of operations in response to said program; selecting a set of operations from said sequence of operations; monitoring respective operations of said set of operations; recording access information associated with respective operations of said set of operations and identifying one or more memory objects being accessed by respective ones of said set of operations; enabling a watchpoint mechanism operable to detect one or more next operations accessing one or more of said memory objects; and recording access information about said next operations, the access information recorded includes an estimation of a duration between respective ones of said set of selected operations and corresponding ones of said next operations.
 2. The method according to claim 1 wherein said step of recording access information further includes recording at least one of: an identity of one or more of said monitored operations; an identity one or more of said next operations; and an identity of said one of more memory objects being accessed.
 3. The method according to claim 1 wherein said step of enabling a watchpoint mechanism includes enabling a hardware device to provide a signal indicative of said next operations accessing said one or more of said memory objects.
 4. The method according to claim 1 wherein said step of enabling a watchpoint mechanism includes enabling an externally provided trap mechanism to detect said one or more next operations accessing said one or more of said memory objects.
 5. The method according to claim 1 wherein said step of recording further includes recording an association between respective ones of said set of operations and their corresponding next operations.
 6. The method according to claim 5 further comprising the step of recording the context identifier (CID) of respective ones of the said associated operations.
 7. The method according to claim 1 wherein said selecting step further includes enabling an imprecise hardware trap with a maximum skid of S operations to signal S operations before one of said next operations is executed, and to advance the execution using a technique having less skid, until said one next operation is reached.
 8. The method according to claim 1 wherein said step of selecting the said set of operations to be monitored includes executing a phase detection algorithm.
 9. The method according to claim 1 wherein a plurality of said steps are implemented by modifying the stream of instructions the first time said instructions are executed in the said sequence.
 10. The method according to claim 1 wherein information about several different memory object sizes are monitored in parallel for each of said operations of said set of operations.
 11. The method according to claim 1 wherein said step of selecting a set of operations includes dynamically adjusting a ratio of operations to be monitored such that said ratio is increased the longer the program is running.
 12. The method according to claim 1 wherein said step of recording access information about said selected set of operations further comprises recording one or more pieces of information selected from the group consisting of: identities of said selected operations; identities of said memory objects; memory addresses being accessed; identities of memory operations following next after respective ones of said selected set of operations; and identities of memory operations preceding before respective ones of said selected set of operations.
 13. The method according to claim 1 wherein said step of recording access information about said next operations further comprises recording one or more pieces of information selected from the group consisting of: identities of said next operations; identities of said memory objects; memory addresses being accessed; identities of memory operations following next after respective ones of said next operations; and identities of memory operations preceding before respective ones of said next operations.
 14. The method according to claim 1 wherein said sequence of operations is executed and the instructions of said program of instructions constitutes contents of said memory objects.
 15. The method according to claim 1 further including estimating an amount of memory objects used by the sequence, of instructions including the steps of: estimating a fraction of said monitored instructions for which no next instruction is detected; and multiplying said fraction with a total number of said operations executed by said sequence of instructions.
 16. A method of estimating a cache miss rate for a cache system, the method executed by a processor and including the steps of: executing a sequence of accesses to a memory system; recording reuse distance information for a selected fraction of said memory accesses of said sequence; solving an unknown miss rate variable from an equation equating the total number of memory accesses multiplied by the said miss rate, with a number of unique memory objects in the said sequence plus a sum of the miss probabilities for the said selection fraction of memory accesses divided by the said fraction; and obtaining the cache miss rate by subtracting a cold miss rate from a value of said miss rate variable.
 17. The method according to claim 16 wherein said step of solving an unknown miss rate variable includes modeling a miss probability of second memory access as a function of miss rate, reuse distance, cache size and cache line size.
 18. The method according to claim 16 wherein said step of solving for an unknown miss rate variable includes estimating a miss probability including the steps of: calculating a probability that a memory object remains in a cache after one cache replacement, as one minus the inverted number of possible locations in the cache that the said memory object can occupy; calculating the expectation of the number of cache replacement between two subsequent accesses to the same memory object as the reuse distance times the miss rate; and calculating the probability that a memory object does not remain in the cache after said expectation of the number of cache replacements, as one minus the said cache miss probability after one miss raised to the power of the said expectation of the number of cache replacements.
 19. A method of estimating a number of unique memory objects accessed for a segment of a program of instructions including the steps of: executing said program and estimating a representative reuse distance distribution for memory operations of said segment; estimating an amount of memory operations performed during said segment; for each natural number between said amount and the number one, using said reuse distance distribution to calculate a probability a memory operation of said distribution has a reuse distance larger than said natural number; adding all said probabilities together; and estimating a miss rate of a cache with LRU replacement strategy, including finding, a smallest reuse distance value for which a number of unique memory objects is larger than the number of memory objects that can be stored in the cache; and calculating a fraction of said memory operations of said reuse distribution with a reuse value larger than said number of memory objects that can be stored in the cache.
 20. An apparatus for gathering memory usage information for a program of instructions, said apparatus comprising: a processor operating to execute a sequence of operations in response to said program; selecting logic to select a set of operations from said sequence of operations, the selecting logic including logic to dynamically adjust a ratio of operations to be monitored such that said ratio is increased the longer the program is running; monitoring logic to monitor respective operations of said set of operations; a first memory to store access information associated with respective operations of said set of operations including an identify one or more memory objects being accessed by respective ones of said set of operations; a watchpoint mechanism operable to detect one or more next operations accessing one or more of said memory objects; and a second memory to store access information about said next operations.
 21. The apparatus according to claim 20 wherein said first memory stores said access information about said next operations including one or more pieces of information selected from the group consisting of: identities of said next operations; identities of said memory objects; memory addresses being accessed; identities of memory operations following next after respective ones of said next operations; and identities of memory operations preceding before respective ones of said next operations.
 22. A non-transitory computer readable medium comprising code for gathering memory usage information, said code comprising instructions for: starting to execute a sequence of operations in response to said program; selecting a set of operations from said sequence of operations; monitoring respective operations of said set of operations; recording access information associated with respective operations of said set of operations and identifying one or more memory objects being accessed by respective ones of said set of operations; enabling a watchpoint mechanism operable to detect one or more next operations accessing one or more of said memory objects; recording access information about said next operations; and estimating an amount of memory objects used by the sequence of operations, the estimating including: estimating a fraction of said monitored instructions for which no next instruction is detected; and multiplying said fraction with a total number of said operations executed by said sequence of instructions. 